The present invention includes methods and apparatus for making connections to an integrated circuit chip. The Demountable Tape-Automated Bonding System provides a modular, reliable, and self-aligned device that overcomes the main drawbacks of conventional chip interface technology.
Each year integrated circuits become more powerful and capable of storing more information. One of greatest challenges confronting designers in the electronics industry is finding more efficient and reliable methods to access the complex circuitry that resides within the chip package. A large rectangular IC chip having sides less than one half inch in length may have as many as five hundred or more leads extending from it. Each of these conductors must be physically bonded or coupled to external devices. These conductors are spaced together so closely that the center-to-center distance, or "pitch", across a pair of leads may measure less than five mils. As the leads become finer and finer, and as they are arranged in more tightly packed arrays, chances for mechanical failure and electrical short circuits quickly rise to unacceptably high levels.
The basic methods of mass-producing connections to memory or logic chips utilize a technique called "tape-automated bonding". This fabrication procedure is commonly referred to by the acronym "TAB" and is well known to persons ordinarily skilled in the electronics packaging art. A continuous insulative tape which is similar to photographic film provides a planar support for chips that are attached to individual sections or frames of the tape. TAB frames are generally rectangular or square portions that are arranged side-by-side along an uncut tape. A spider-like metal pattern of conductive traces is formed on each frame. The traces radiate from the center of the frame to its four edges. A chip is carefully aligned over the center of the TAB frame, so that the leads or contacts of the chip are precisely mated with the corresponding metal traces in the central portion of the TAB frame. In previous integrated circuit devices, the chip is attached to the TAB frame with a bonding agent. This connection of the chip contacts to the inner portion of the TAB frame is referred to as "inner-lead bonding" or "ILB" technology. The outermost sections of the conductive traces on the TAB frame must also be carefully aligned with a similar set of corresponding traces formed on a substrate. The substrate is attached to the TAB frame on the side that is opposite from the side that is adjacent to the chip. The connection of the outer portion of the TAB frame to a substrate like a printed circuit board pertains to "outer-lead bonding," often called "OLB" technology. The resulting assembly comprising the chip, the TAB frame, and the substrate is essentially a space transformer that employs diverging radial electrical pathways to afford ready access to the integrated circuit.
Conventional manufacturing operations utilize soldering or thermocompression equipment to form metallurgical junctions between the chip contacts and the planar traces as well as between the TAB traces and the substrate. Great quantities of heat and pressure must be applied to the chip, TAB frame, and substrate during these mating processes. Excessive heat and pressure can seriously damage or even destroy the fragile circuitry within the chip. Highly accurate lasers capable of supplying radiant energy with great precision can be employed to diminish the chances for damage due to stray heat, but the basic problem of keeping the chip out of harm's way during a rapid automated manufacturing operation still remains unsolved. The heat expended during conventional procedures also tends to cause the metal and plastic components within the assembly to expand. The resulting deformations can easily ruin the registration of the contacts and the traces. Even if engineers and technicians can manage to coax acceptable yield rates from these methods of fabrication, the finished product is virtually impossible to repair or to service. As chips become more complicated and more expensive, it becomes less and less desirable to throw away entire chip, TAB frame, and substrate assemblies when a breakdown is encountered.
The problem of providing reliable chip-to-TAB frame and TAB frame-to-substrate connection systems that avoid the detrimental side effects that inherently accompany soldering and thermocompression bonding has presented a major challenge to designers in the automated electronic packaging field. The development of a relatively low cost, rugged, and versatile system that does not suffer from the impediments of misalignment and the inability to repair and replace defective chips would constitute a major technological advance in the electronics business. The enhanced performance that could be achieved using such an innovative device would satisfy a long felt need within the industry and would enable electronic equipment manufacturers to save substantial expenditures of time and money.